The present invention relates to a high-voltage pulse generating apparatus for linear accelerators, radars, excimer lasers, etc., which has at least one magnetic switch comprising a magnetic core made of an Fe-base soft magnetic alloy.
Pulses applied to such apparatuses as linear accelerators, excimer lasers, etc. have as extremely narrow widths as several tens n sec to several hundreds n sec, and it is necessary to use a pulse generator capable of generating high voltage more than several tens of kV. Further, energy of a single pulse is as large as several tens of joules or more and the repetition of such pulses is as much as 1 kHz or more. Under such severe conditions, the high-voltage pulse generating apparatus should be operated stably.
Conventionally used as a switch for high-voltage pulse generators is a thyratron and a spark gap, but their service lives are extremely short when used to generate high-power, narrow-width pulses as described above.
An alternative to the above apparatuses is a pulse compression circuit containing at least one magnetic switch constituted by an amorphous alloy magnetic core as shown in FIG. 1 [Japanese Patent Laid-Open Nos. 59-63704 and 60-96182, U.S. Pat. No. 4,275,317. etc.]. FIG. 1 schematically shows a 3-step pulse compression circuit containing 3 magnetic switches S.sub.1, S.sub.2 and S.sub.3, but the use of n magnetic switches can provide a n-step pulse compression circuit with the same principle. In FIG. 1, to increase an energy transmitting efficiency, C.sub.1 should be C.sub.2, and the magnetic switches S.sub.1, S.sub.2 and S.sub.3 should have successively decreasing inductance.
In FIG. 1, when first capacitor C.sub.1 reaches a predetermined high voltage V.sub.1, a switch SW is closed. At this time, current I.sub.1 is extremely small because the magnetic switch S.sub.1 has high impedance. However, when the magnetic switch S.sub.1 becomes saturated, its impedance becomes extremely small. As a result, the charge in the capacitor C.sub.1 flows into the second capacitor C.sub.2 instantaneously, making the current I.sub.1 extremely large in a very short time. A core constant of the magnetic switch S.sub.2 is determined such that the magnetic switch S.sub.2 can retain high impedance until the second capacitor C.sub.2 is fully charged. Next, when the second capacitor C.sub.2 reaches fully high voltage, a magnetic core of the second magnetic switch S.sub.2 becomes saturated, permitting the charge of the second capacitor C.sub.2 to flow into a pulse forming line (PFM). By repeating this action successively, the pulse is compressed as shown by I.sub.1, I.sub.2, I.sub.3, and the compressed pulse I.sub.3 is applied to a load 1. The compression of pulse is graphically shown in FIG. 2.
The magnetic core used for such a magnetic switch is required to have the following properties.
First, the magnetic switch operable in this manner is magnetized according to the relation derived from Maxwell electromagnetic equations: EQU VT=NS.DELTA.B. (1)
V: Voltage applied to a magnetic switch. PA1 T: Time during which the voltage is applied. PA1 N: Number of winding of a magnetic switch core. PA1 .DELTA.B: Variation of magnetic flux density. PA1 (a) The core should have as large a squareness ratio as possible and its relative permeability after saturation should be as close to 1 as possible. PA1 (b) The magnetic core should have as small a core volume as possible, and the inductance of the central space should be as small as possible. This condition is substantially the same as the above first condition. PA1 A.sub.e : Effective cross-section of core 4, and PA1 E.sub.r : Voltage of power source 5, PA1 E.sub.g : voltage of power source 7,
Therefore, under the conditions that N and VT are constant, the larger .DELTA.B the smaller S, which means that the cross section of the core can be reduced as much as possible, because a magnetic core volume is proportional to 1/(.DELTA.B).sup.2. Here, the product of VT is determined from the condition that the second magnetic switch S.sub.2 has high impedance until the second capacitor C.sub.2 is fully charged. FIG. 3 schematically shows the magnetization of a magnetic switch core. Since the magnetic flux of the core changes along the line (b) from a starting point -B.sub.r, the larger .DELTA.B (B.sub.r +B.sub.s), the more desirable the magnetic core, which means that a core material having larger saturation magnetic flux density B.sub.s and squareness ratio (B.sub.r /B.sub.s) is more preferable.
Second, it is desirable that the magnetic switch has as large inductance L.sub.r in an unsaturated region and as small L.sub.sat in a saturated region as possible. This is because the compression of the pulse is proportional to (L.sub.sat /L.sub.r).sup.1/2.
To decrease L.sub.sat, the following points are important:
To increase L.sub.r, it is important to increase the permeability in an unsaturated region, and to reduce a length of a magnetic path in the core. With respect to the magnetic core material, it is important that it has a small core loss at high frequency, because if otherwise H.sub.c becomes large and the gradient of the line (b) in FIG. 3 which indicates .mu..sub.r =.DELTA.B/H.sub.s becomes small. It is also important that the magnetic core has large .DELTA.B. Further, the magnetic core should have as small cross-section as possible.
Third, the variation of the above properties with time should be as small as possible.
In sum, with respect to the core material used for a magnetic switch, it is important that it has a large saturation magnetic flux density B.sub.s, a large squareness ratio B.sub.r /B.sub.s, a small core loss at a high frequency and small variation of magnetic properties with time.
For such requirements, amorphous alloys are highly suitable, and they have been used conventionally. Typical amorphous alloys have such properties as B.sub.s, .DELTA.B, .mu..sub.r, core loss as shown in Table 1.
TABLE 1 __________________________________________________________________________ Sample Composition Heat Treatment B.sub.s .DELTA.B Core Volume Core Loss No. (atomic %) Temperature (.degree.C.) (T) (T) .mu..sub.r (Relative Value) (Relative Value) __________________________________________________________________________ 1 Fe.sub.71 B.sub.13.5 Si.sub.13.5 C.sub.2 360 1.60 2.90 1400 1 4.35 2 Fe.sub.78 B.sub.13 Si.sub.9 400 1.55 2.84 360 1.04 17.1 3 Fe.sub.79 B.sub.16 Si.sub.5 420 1.58 2.21 4200 1.72 1.45 4 Fe.sub.73 Ni.sub.5 Si.sub.13 B.sub.9 400 1.44 2.75 1570 1.11 3.87 5 CO.sub.73.3 Fe.sub.0.7 Mn.sub.3 Si.sub.14 B.sub.9 220 0.83 1.57 6090 3.41 1 6 Mn--Zn Ferrite -- 0.48 0.72 5700 16.22 1.09 __________________________________________________________________________ as follows:
The core loss is evaluated by a circuit shown in FIG. 4. FIG. 5 shows wave forms of voltage and current in various parts in the circuit in FIG. 4, and FIG. 6 the magnetization process of the magnetic core being evaluated.
In FIG. 4, when a semiconductor switch 1 is turned on, voltage e.sub.r as shown in FIG. 5 appears in a winding 2 in an opposite polarity to that shown by a dot in FIG. 4. Assuming that: ##EQU1## T.sub.r : Turn-on period of switch 3, N.sub.r : Number of winding of 2,
the magnetic core 4 is saturated at -B.sub.s in the third quadrant along the B-H loop in FIG. 6. Next, assuming: EQU T.sub.p &gt;&gt;T.sub.r, (3) EQU T.sub.p : period,
the magnetic flux density of the magnetic core 4 immediately before turning on the main switch 1 of a gate circuit is -B.sub.r, a residual magnetic flux density in the B-H loop in FIG. 6. Next, when the main switch 1 is turned on, ##EQU2## T.sub.on : Turn-on period of switch 1, N.sub.g : Number of winding of 6, and
the magnetic core is saturated and magnetized to: ##EQU3## I.sub.gm : Maximum wave height of gate current i.sub.g, and l.sub.e : Average magnetic path length of core 4.
In the above process, the magnetic core 4 is magnetized along the solid line in FIG. 6 during a period Ton between turn on and turn off of the main switch 1. Here, the following relation exists: ##EQU4## N.sub.s : Number of winding of search coil.
On the other hand, as is clear from FIG. 6, ##EQU5##
In addition, the magnetic core loss of a single pulse per unit volume is: ##EQU6##
The total core loss P.sub.ct of a magnetic core is related to P.sub.c as follows: EQU P.sub.ct =A.sub.e .multidot.l.sub.e .multidot.P.sub.c ( 9)
In addition, the following relation generally exists: EQU A.sub.e .multidot.l.sub.e .varies.(1/.DELTA.B).sup.2 ( 10)
The substitution of (9), (10) into (8) leads to the relation: ##EQU7##
This means that the larger .mu..sub.r, the smaller P.sub.ct. Accordingly, by measurement with this evaluation circuit, it is verified that the larger .DELTA.B, the smaller a saturable magnetic core, and that the larger .mu..sub.r, the smaller a total magnetic core loss P.sub.ct /f of a single pulse.
Incidentally, cores shown in Table 1 are those constituted by amorphous alloy ribbons having a thickness of about 50 .mu.m, with a polyimide insulating tape of 9 .mu.m in thickness interposed between adjacent amorphous alloy ribbons. Each core has an outer diameter of 100 mm, an inner diameter of 60 mm and a height of 25 mm. Each magnetic core is heat-treated at an optimum temperature while applying a magnetic field of 800 A/m in parallel with each magnetic path. For comparison, a Mn-Zn ferrite core having the same size is also measured, and its data are shown in Table 1. Incidentally, by utilizing the relation that the core volume is proportional to 1/(.phi.B).sup.2, the core volume of each magnetic core is calculated by using .DELTA.B, assuming that the core volume of No. 1 core is 1.
As is clear from Table 1, the ferrite core shows much smaller core loss than the amorphous alloy core (No. 1) , but the ferrite core's volume is about 16 times as that of No. 1 core because of small .DELTA.B. Of course, since an amorphous alloy core has a small space factor (a ratio of amorphous alloy ribbon to an apparent volume of the core), its actual volume is not as large as shown in Table 1. But even if No. 1 core has a space factor of 0.60, the ferrite core is as large as about 6 times.
As is clear from Table 1, the amorphous alloys show better properties than the ferrite as core materials for magnetic switches, but the amorphous alloys having small core volumes show large core losses and vice versa. Thus, there are no amorphous alloy core materials with a good balance of magnetic properties. Specifically speaking, amorphous alloys are classified into Fe-base alloys and Co-base alloys, and the Fe-base amorphous alloys have large B.sub.s and core losses, while the Co-base amorphous alloys have small core losses and B.sub.s. Because of this, the amorphous alloy cores developed heretofor are not necessarily satisfactory.
In addition, the amorphous alloys do not have sufficient stability with time.